1. Field
One embodiment of the present invention relates to a disk drive. More particularly, the invention relates to a technique of updating the disk-runout information whenever a disk shift occurs in the disk drive.
2. Description of the Related Art
In most disk drives, a representative example of which is the hard disk drive, head-positioning control is performed to move the magnetic head to a target position over the disk (i.e., target track or target cylinder). The magnetic head has a read head and a write head. The read head and write head are mounted on a slider and spaced apart from each other. The read head is configured to read data recorded in the disk. The write head is configured to write data in the disk.
The disk drive has a rotary-type actuator, which holds the magnetic head. When controlled and driven, the actuator moves the magnetic head, positioning the magnetic head at the target position over the disk. More precisely, the write head must be positioned at the target position in order to record data in the disk, and the read head must be positioned at the target position in order to reproduce data from the disk. Since the read head and the write head are located at different positions as described above, the magnetic head must be subjected to position adjustment called “offset” so that it may record and reproduce data reliably.
The position adjustment of the magnetic head is accomplished by using the offset values stored in a memory incorporated in the disk drive. The offset values have been determined from the distance between the read and write heads and from the skew angle of the actuator. (The skew angle changes while the magnetic head is being moved in the radial direction of the disk.)
During the manufacture of the disk drive, so-called “disk runout” develops. The disk runout occurs as the spindle motor rotates the disk. If the disk is eccentric to the shaft of the spindle motor, the physical position the magnetic head has with respect to the same track changes every time the disk rotates 360°. As the physical position of the magnetic head changes, the offset value changes, too.
Any disk drive in which disk runout develops needs to perform so-called dynamic offset control (DOC) function of correcting the offset value by using, as parameters, the absolute value and phase of disk runout with respect to each track provided on the disk. The DOC function can suppress the head-positioning error deriving from the disk runout.
The disk runout is measured during the manufacture of the disk drive and is stored in the memory incorporated in the disk drive. After the disk drive has been shipped, however, a phenomenon called “disk shift” (i.e., shift of the rotation center of the disk) takes place when the disk drive receives an external impact. The disk shift changes the disk runout that should be applied to the DOC function. In view of this, the disk drive should preferably have a function of measuring the disk runout and updating the disk-runout information stored in the internal memory.
For example, Japanese Patent No. 3198490 proposes a method of updating disk-runout information is proposed. In this method, a disk runout is calculated anew from the changes in the interval of the servo data items recorded in the disk (i.e., servo-interval change) and is stored the memory incorporated in the disk drive.
As described above, an accurate disk runout is indispensable to the DOC function in the disk drive. It is therefore important for the disk drive to measure the disk-runout accurately at all times and to update the disk-runout information stored in the internal memory. However, the method disclosed in the above-identified publication cannot calculate accurate disk runouts. This is because the servo-interval change mentioned above is influenced by not only the changes in disk rotation speed, but also by changes in temperature.